Tonometers are known in the art and are widely used to facilitate indirect measurement of the intraocular pressure of a person's eye. The ability to obtain measurements of intraocular pressure has proved important for detecting diseases, such as glaucoma, or other ailments of the eye.
Various types of tonometers have been developed and may be classified, generally, as non-contact tonometers, indentation tonometers and applanation tonometers. Non-contact tonometers generally utilize a puff of air applied to the cornea of an eye in conjunction with shining a light beam onto the eye. As the cornea becomes flattened by the puff of air, the light intensity reflected from the cornea of the eye varies and is detected by an optical device. Non-contact type tonometers are generally considered by medical professionals to be less accurate than contact-type tonometers and thus are primarily used for initial screening purposes.
While non-contact type tonometers do not actually touch the eye, indentation tonometers and applanation tonometers include members that are actually brought into physical contact with the cornea of an eye to depress a portion of the cornea while obtaining a measurement of force. An indentation tonometer utilizes a weighted plunger to apply a known force to the cornea while measuring the deformation produced. An applanation tonometer measures the force required to flatten a predetermined area of the cornea. By far the most widely used mechanical tonometer is the Goldmann applanation tonometer, described in U.S. Pat. No. 3,070,997. Using the Goldmann applanation tonometer, the cornea of an eye being tested is flattened with a probe having a 3.06 mm diameter flat area. Once the cornea has been flattened to the 3.06 mm diameter, the force required to flatten the cornea is measured and used to calculate the internal pressure of the eye.
While the Goldmann applanation tonometer is widely used and familiar to practitioners, it also suffers several drawbacks. For example, the Goldmann-type tonometer is a complex mechanical system comprising internal weights, springs and bearings which must be periodically calibrated to ensure accurate measurements of intraocular eye pressure. This drawback is heightened by the fact that there are relatively few persons who have the resources and technical capability for performing these periodic calibrations. Furthermore, the Goldmann applanation tonometer is susceptible to error due to improper balancing or level orientation of the tonometer with respect to the microscope and is sensitive to interference from electromagnetic fields. Accordingly, there is thus a need for an applanation tonometer which overcomes drawbacks of the prior art such as those described above.